Surge on compressors is a serious problem, causing severe vibration, and sometimes damage to the rotating equipment. A typical surge control system as known in the prior art is shown in FIG. 1 in which motor 5 drives compressor 10. Gas flows into compressor 10 along pipe 15 into low pressure input suction pipe 20 and is discharged along pipe 25 into gas output pipe 30. Gas flow is measured and monitored by surge controller 35 along line 40 while suction pressure, if this is known to change, is measured and monitored along line 45 and discharge pressure is measured and monitored along line 50. In simplified terms, surge results from excessive pressure in the compressor discharge along pipe 25 causing the gas flow to reverse back into compressor 10. Surge is controlled with a large bypass valve 55 which allows high pressure gas at the discharge to flow back into low pressure suction pipe 20, thus relieving the pressure. Bypass valve 55 is controlled by surge controller 35 along signal line 60. Other devices besides a simple bypass valve may also be used. For example, a spring-loaded blow-off valve which dumps high discharge pressure gas to a flare (burner) is a possibility. However, such a valve has to be set low and is wasteful and expensive if used very much. A third similar mechanism is a spring-loaded relief valve on the discharge which opens when the pressure reaches a preset high value and dumps gas back to the suction. This type of valve also has to be set low to be safe and has the same drawbacks as previously described.
During factory testing of compressor 10, pressure and flow are measured at a range of rotating speeds, and the onset of surge carefully measured and drawn on a two-dimensional Surge Map in graphical form displaying the relationship between input gas flow and discharge pressure. For each pressure and flow point, the corresponding horsepower and motor speed are measured and drawn on the map, as shown in FIG. 2. This map is used to program surge controller 35 so that it opens bypass valve 55 if flow conditions approach the surge limit as shown in FIG. 2. If the suction pressure can also change in the particular configuration, the vertical axis of FIG. 2 is modified so as to represent a pressure ratio (discharge pressure/suction pressure) and thus create a similar shaped set of curves which are used for surge control.
During normal compressor operation, operating point X is in the safe operating area which is that area to the right of control line A, as shown in FIG. 3 which presents a surge map in graphical form of the relationship between compressor suction flow on the horizontal axis and compressor discharge at various operating points on the vertical axis. Control line A itself is programmed into the controller a discretionary amount to the right of surge line B to provide a margin of safety in system operation. If, due to changing process conditions or compressor speed change, the operating point moves across control line A, controller 35 starts to open bypass valve 55 to relieve the discharge pressure and bring compressor 10 back into the safe operating area. FIG. 3 also shows the relationship of compressor suction flow to compressor discharge pressure at various exemplary constant speeds C, D, E, F and G, in RPM of motor 5. As those constant speeds increase, the relative suctions flow and pressure also increase.
Surge controller 35 has a Proportional plus Integral (PI) controller 100 having as inputs at 115 the operating point from the surge controller map and, at 120, the set point from the control line from the surge controller map. PI 100 develops a current output 105, as shown in FIG. 4. The current output is converted by a converter at 110 to either air pressure or hydraulic pressure to actuate bypass valve 55, which is normally closed. When a control error is detected, PI 100 generates an output which starts to open valve 55 to allow bypass flow. As the error diminishes, the output decreases, and the valve closes. Adaptive control can be added to the PI to control for special conditions.
The traditional surge control system described above requires at least three sensors to measure gas flow, suction pressure and discharge pressure as well as a microprocessor-based surge controller. A simpler, more reliable, and less expensive surge control system would be a benefit to users.